It solves everything, it's fair, DC residents would also gain representation in the Senate, and I've never heard an argument against it.
Does anybody have one?
1. Neither Maryland nor D.C. want to be in that marriage.
2. D.C. has been an independent political entity for a lot longer than most states. North Dakota and Delaware as independent states doesn't make much sense, but we're stuck with them because of history and they've developed their own regional identities.
3. D.C. would lose self-government by being subordinated to Maryland.